1. Field of the Invention
This invention relates to room-temperature-vulcanizable (RTV) organosilicon compositions. More particularly, this invention relates to RTV organosilicon compositions containing a specified class of dialkoxysilanes. The resultant cured silicone rubber products can be readily coated with paint.
2. Description of the Prior Art
One part RTV organosilicon compositions are stable under conditions that exclude moisture. In the presence of atmospheric moisture the compositions form cured silicone elastomers at ambient temperature. One type of one part RTV composition contains a hydroxy terminated polyorganosiloxane and, as the curing agent, an amide-substituted organosilicon compound, an aminoxy-substituted organosilicon compound, or mixtures thereof. In this type of composition both a difunctional organosilicon compound and one containing three or more functional groups per molecule are used as curing agents. By varying the proportion of these two types of curing agents, the physical properties of the cured elastomer can be varied over a broad range, for example, from low modulus/high elongation to high modulus/low elongation. The low modulus/high elongation elastomers are particularly useful as architectural sealants due to their excellent resistance to weather and fatigue.
A serious disadvantage of the aforementioned sealants is that the silicone rubber surface obtained following curing of the sealant either cannot be easily painted, or the paint is only weakly adhered to the surface.
European patent application No. 50453, published on Apr. 28, 1982, discloses room temperature vulcanizable compositions containing silanes or siloxanes with at least one aminoalkyl group and at least one alkoxy group per molecule as adhesion promoters. No distinction is made between compounds containing 1, 2 or 3 alkoxy groups. The examples employ only compounds containing 3 alkoxy groups or 2 alkoxy groups and one siloxy group bonded to silicon in addition to the aminoalkyl groups. No dialkoxysilanes are employed in the examples, although .gamma.(.beta.-aminoethylamino)-propylmethyldimethoxysilane is disclosed, as is .delta.-aminobutylmethyldiethoxysilane. The RTV compositions also include an organosilicon compound with at least 3 hydrolyzable groups bonded to silicon, which functions as the curing agent for an essentially linear polyorganosiloxane containing terminal groups that are hydroxyl or readily hydrolyzable groups. Aminoxy and amido are among the hydrolyzable groups that can be present on the curing agent.
The prior art does not disclose cured silicone rubbers that can be painted and which are prepared by curing RTV compositions containing either dialkoxysilanes having reactive groups such as amino, acryloxy, epoxy and mercapto that are bonded to silicon by means of alkylene radicals, or hydrolysis condensation products of these dialkoxysilanes.
U.S. Pat. No. 3,812,164, which issued on May 21, 1974, and U.S. Pat. Nos. 3,836,502 and 3,836,503, both of which issued on Sept. 17, 1974, disclose imparting paintability to silicone elastomers by including certain substituted mercaptoalkylsilanes of the general formula (RO).sub.3 Si(CH.sub.2).sub.x SY, where x is 1, 2, 3, or 4. Y represents ##STR1## in U.S. Pat. No. 3,812,164; a residue derived from nadic anhydride or nadic methyl anhydride in U.S. Pat. No. 3,836,502; and --CH.dbd.CHC.sub.6 H.sub.5 in U.S. Pat. No. 3,836,503. Each of the aforementioned substituted mercaptoalkylsilanes are prepared using a mercaptoalkyltrialkoxysilane as one of the reactants, and would therefore be most costly to produce than the mercaptoalkylsilane itself. No dialkoxysilanes are disclosed in these 3 U.S. patents.
It has now been found that certain dialkoxysilanes or hydrolysis condensation products thereof that contain at least one silicon-bonded group of the formula --R.sup.1 X' will impart paintability to silicone rubbers prepared using organosilicon compounds containing amido and/or aminoxy groups as the curing agent. In the foregoing formula R.sup.1 represents a divalent radical selected from the group consisting of alkylene, haloalkylene, and oxyalkylene and X' represents an amino, substituted amino, mercapto, methacryloxy, acryloxy, or epoxy group.